Duchenne Muscular Dystrophy (DMD) is an inherited, sex-linked disease characterized by progressive degeneration of the skeletal musculature. A number of different lines of evidence point to the involvement of cellular membranes in the primary etiology of the disease. A major problem with studies aimed at defining the primary etiology of DMD has been the difficulty of distinguishing between primary and secondary changes in the degenerating muscle tissue. To avoid this problem we have studied the biochemistry of cultured fibroblasts from patients with DMD. We have recently identified in these cells three lysosomal abnormalities: (1) a marked reduction (p less than 0.005) in the activity of dipeptidyl aminopeptidase I (DAP-I), a chloride-requiring, lysosomal enzyme; (2) a pronounced decrease (p less than 0.001) in the membrane-linked, latency of DAP-I in DMD cells; (3) associated with these two abnormalities, an abundance of cytoplasmic lamellar bodies in the DMD cells (p less than 0.01). We believe that these observations demonstrate the expression of the dystrophic gene, albeit in a non-pathological manner, in these non-muscle cells. Based on these observations, the immediate goals of this study are threefold. First, to test the hypothesis that the decreased DMD DAP-I levels may be due to an underlying increase in membrane chloride permeability or in cellular chloride concentration. Second, to test the alternate hypothesis that the DAP-I decrease may be due to an altered rate of enzyme turnover. And third, closely related to this possibility, to answer the more general question of whether lysosomal enzymes are resistant to autoproteolysis and, if so, why. The long-range objectives of this research are to define the primary etiology of this common muscular dystrophy and to understand the mechanism by which this defect specifically disrupts the normal processes of muscle cell development.